The invention relates to a method for detecting slow and small changes of electrical signals including the sign of the changes, to determine the end-of-charge condition of batteries being charged.
Direct current voltages can generally be measured with a required accuracy. There are, however, special tasks of measurements, wherein changes should be exactly detected, which are very low relative to the level of the direct current voltage, e.g. 10-3 or 10-4 times of the DC level, and such changes take place slowly, e.g. during a couple of hours. The difficulty of the task increases if the occurrence of such slow changes should be detected very fast that means less than a couple of minutes, and the detection time might be in the order of magnitude of 10 seconds. In case of such detection tasks conventional methods of measuring voltages cannot be used, since the useful signal is not higher than the accuracy of the measurements.
Typically such a task is the determination of the end-of-charge moment in case of charging batteries. Especially, when the battery is charged intensively with a high charging current, the charging process would be finished as soon as the fully charged state has been reached, otherwise the battery might suffer an irreversible damage.
In the booklet of Motorola Inc. SG 73/D Rev. 17, 1998 of the Master Selection Guide series, an integrated battery charger circuit type MC 33340P is described that can detect the decrease of the battery voltage by a sensitivity of 4 mV. Useful accuracy, for the objectives of the invention, may be much higher than this value, and it may not be sufficient to detect the decrease of the voltage only, it is desirable to determine the tendency of the change as well. Determining the tendency means making a determination as to whether the signal has decreased by a predetermined extent, whether it has increased at least by that extent or whether it has remained unchanged i.e. the fluctuations have not exceeded the predetermined level. The battery voltage starts decreasing after it has reached its maximum. There are several types of batteries, wherein there is no sudden maximum but rather a steady voltage condition through a longer period of time, and the life of the battery gets reduced when it is charged throughout that voltage plateau.
U.S. Pat. No. 4,137,493 describes a detector circuit for detecting changes in the level of a DC voltage used for controlling the end-of-charge moment of a battery charger. In this detector a capacitor is charged in sampling periods to the DC voltage, and in each sampling period if the DC voltage level has changed since the previous period, a charging or discharging current will flow through the lead out wire of the capacitor until it takes the new DC value. This transient current is monitored and compared with a reference voltage. The accuracy of this detector is limited by the non-compensated DC offset of the applied circuitry.
In case of very small changes of voltage signals, prior to the present invention, there was no kind of reliable and accurate means available to detect the steepness of the changes or the persistence of an unchanged state of the signal. The knowledge of such parameters would be, however, desirable in several applications of the invention.
Furthermore, the invention provides a method for determining an end-of-charge moment for a battery being charged, the method comprising the steps of:                periodically sampling an electrical parameter of the battery, said parameter being selected from the group consisting of the battery voltage and the charging current;        determining at each sampling the change in the value of said electrical parameter compared with the value taken in the immediately previous period; and        generating an end-of-charge signal when said change in value lies below a predetermined threshold level of change.        